1. Field of the Invention
This invention relates to a process for manufacturing high-strength parts of an automobile, particularly of its transmission system.
2. Description of the Prior Art
Hot forging, casting, sintering, etc. have hitherto been employed for making plate carriers and other parts of automatic or other transmission systems in automobiles. Press forming and soft-nitriding or other heat treatment have recently come to be employed for making materials of higher strength to enable the manufacture of automobiles which are lighter in weight and less expensive, and yet ensure a higher level of safety for the driver or passenger. High strength is essentially required of, among others, certain parts including the plate carrier of an automatic transmission.
The manufacture of a high-strength part by press forming necessitates the use of a sheet material having a relatively large thickness in the range of, say, 2 to 6 mm. A hot-rolled steel sheet is usually employed. Attempts have been made to use a hot-rolled steel sheet having a high strength which is equivalent to the strength required of a final product. A high-strength hot-rolled steel sheet is, however, low in press workability and causes a heavy wear to the tool used for its working, and is, therefore, unsuitable for use in the commercial production of any such part.
There is also known a method in which a part formed from mild steel is carburized, nitrided, soft-nitrided, or otherwise treated to acquire a surface having the desired strength and hardness. The product of this method is, however, low in rigidity, particularly in torsional strength and fatigue resistance, since it is not strong enough in its interior as opposed to its surface. There is no alternative but to use a sheet having a larger thickness or rely to a greater extent upon surface-hardening treatment in order to make up for any such drawback. This is contrary to the intention to achieve a reduction in the weight and cost of any such part.
Attempts have been made to overcome these problems by forming a part from carbon steel having a relatively low strength and subjecting it to heat treatment (hardening and tempering). The heat treatment, however, calls for the use of a considerably high temperature in the order of at least 850.degree. C. and necessarily adds greatly to the cost of manufacture including not only the cost of heat treatment itself, but also the cost of rectifying any deformation of the part that may result from its heat treatment.
There are also known methods which rely upon special work to increase the strength of steel, as disclosed in, for example, Japanese Patent Publications Nos. 5616/1976 and 17049/1982. All of these methods have, however, been found only capable of achieving a tensile strength which is lower than 80 kgf/mm.sup.2.